alt Hacker NewsAuto-vectorization is consistently one of the least predictable optimization passes, which is rather awful, since when it doesn't trigger your functions are suddenly >3x slower. This drives people to more explicit SIMD coding, from direct assembly like in FFMPEG to wrappers providing some cross-platform support like Google's Highway.
It's just really hard to detect and exploit profitable and safe vectorization opportunities. The theory behind some of the optimizers is beautiful, though: https://en.wikipedia.org/wiki/Polytope_model
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i am quietly waiting for the "bitter lesson" to hit compilers: a large language model that speaks in LLVM IR tokens that takes unoptimized IR from the frontend, and spits out an optimized version that works better than any "classical" compiler.
the only thing that might stand in the way is a dependence on reproducibility, but it seems like a weak argument: We already have a long history of people trying to push build reproducibility, and for better or worse they never got traction.
same story with LTO and PGO: I can't think of anyone other than browser and compiler people who are using either (and even they took a long time before they started using them). judged to be more effort than its worth i guess.
I’m always shocked at what the compiler is able to deduce wrt vectorization. When it works, it’s magical.